Morphology and plasmonic properties of self-organized arrays of ...

99a. b.[001][001]c.[001]d.500 nmFigure 6.2: Top panels: AFM images of a nanopatterned LiF(110) substrate, after thedeposition of t LiF ≈ 250 nm LiF at T = 300 ◦ C (panel (a)), and the grazing depositionof t Au ≈ 5 nm gold at T = 100 ◦ C and subsequent annealing at T = 400 ◦ C (panel (b)).Panel c: AFM image of the same sample after 5’ immersion in solution of Fe 3 O 4 /OAnanoparticles dispersed in hexane (≈ 6μg Fe per ml). Panel d: cross correlation functionof the images in panels (b) and (c); the superimposed grid is a guide to the eye for thepositions of the maxima of the correlation. The Fourier spectral densities for each imageare shown in the insets of the panels.Looking at the AFM image of fig. 6.2(c), the particles appear spherical, their cubicshape being smoothed by the convolution effects of the AFM tip, and the preferentialorientation imposed by the underlying ripple structure can still be recognized. In fact,the characteristic peak of intensity confined in the [1¯10] direction, fingerprint of the LiFripples periodicity, is still present in the Fourier spectrum (inset of the figure), althoughless pronounced than on the bare Au NPs array. Compared to fig. 6.2(b) for the Au NPs,it is less intuitive in fig. 6.2(c) to identify the Fe 3 O 4 /OA NPs arrangement; furthermore,since the typical spacing between the gold NPs is ≈ 33 nm, against a mean Fe 3 O 4 /OANPssizeof≈ 16nm, thereisinprincipleenoughspacetoaccommodate morethanasinglemagnetic NP per Au NP, therefore the arrangement of the Au NPs can hardly be exactlyreproduced in the Fe 3 O 4 /OA layer. Nevertheless, we can try to pick out a correlationbetweentheAuandtheFe 3 O 4 /OANPsarraysbycomputingthecrosscorrelationfunctionof the two previous AFM images; this is shown in fig. 6.2(d). The cross correlation revealsa pattern of maxima and minima, which form an almost square grid with typical spacingsof ≈ 40 nm, comparable to the ripple periodicity and the typical spacings of the Au NPs.